An inertial sensor is a sensor capable of sensing and/or generating motion. An inertial sensor may contain a device of a Microelectromechnical system (MEMS). Examples of such devices include accelerometers capable of sensing acceleration (e.g., MEMS accelerator) and gyroscopes capable of sensing rotation (e.g., MEMS gyroscope). However, a conventional MEMS accelerator is not capable of sensing the rotation, and similarly a conventional MEMS gyroscope is not capable of sensing the acceleration.
Specifically, a typical MEMS accelerator is composed of a static proof mass, springs, and a set of comb fingers attached to it. For example, as shown in FIG. 1, a typical MEMS accelerometer may include a proof mass 102, springs 104, comb fingers 105 and anchors 110. The anchors 110 sit on the substrate 100, and all the other parts are suspended above the substrate 100 and are moveable. When a linear acceleration is applied in the direction of the Y axis, an inertial force generated by the proof mass 102, due to the inertia of the proof mass 102, causes the sensing comb 105 to deform and in turn changes the capacitance of the comb fingers 105. The resulting capacitance change of the comb fingers 105 can then be assessed electronically to obtain acceleration information. Such a conventional MEMS accelerator is not capable of sensing rotation.
A typical MEMS gyroscope is composed of a static proof mass, springs, a set of driving comb fingers and a set of sensing comb fingers. For example, as shown in FIG. 2, a typical MEMS gyroscope may include a proof mass 202, springs 204, movable frames 203, driving comb fingers 206 and sensing comb fingers 205. The anchors 210 sit on the substrate 200, and all the other parts are moveable and are suspended above the substrate 200. During operation, an AC voltage is applied to the driving comb fingers 206, actuating the proof mass 202 into oscillation. More specifically, the electrostatic force generated by 206 drives the moveable frame 203 and the proof mass 202 to move along the X axis in an oscillation manner. When a rotation around the Z axis is applied to the system, a Coriolis force is generated by the moving proof mass 202 and the springs deform in the direction of the Y axis, resulting in a capacitance change of the sensing comb 205. The rotation information can be obtained by assessing the capacitance change of the sensing comb 205 using a readout electronics. The device may be composed of two sub proof masses that move in opposite directions, in order to reduce the rotation signal error caused by the linear acceleration. Such a conventional MEMS gyroscope is not capable of sensing the acceleration.
A MEMS chip can be made by semiconductor fabrication methods and may have single or multiple devices described above. When multiple devices are implemented in a single chip, multiple inertial signals, e.g. rotation and acceleration, or accelerations in multiple axes, can be achieved. Two classes of devices (i.e., gyroscope for sensing rotation and accelerometer for sensing acceleration) are required for a six degree of freedom sensing system. Each class may have a shared device that senses multiple axis information, for example, a single gyroscope that senses two or three axis rotation, and an accelerometer senses two or three axis acceleration.